An optical communication system is used when required data is transmitted between control devices disposed at various points in a press.
In such optical communication system, the following preprocess is performed to operate the system.
First, optical fibers are extended on the spot as optical transmission channels between the control devices. It is then checked whether the extended optical fibers have obtained a desired optical transmission characteristic. The optical fibers are then connected to the control devices to thereby construct the optical communication system.
A preprocess such as extension of the series of the optical fibers or the evaluation of the transmission performance of the optical fibers is conventionally performed as follows.
As shown in FIG. 14, an optical fiber 5 each end of which is already subjected to either required processing or connector attachment is extended between control devices. Optical fiber 5 is then connected at one end to a standard light source 6 such as an LED and at the other end to an optical power meter 8. After the connection, light source 6 is operated, the light emitted by light source 6 is entered into optical fiber 5, and the intensity of the light passing through optical fiber 5 is read by power meter 8. The operator views the scale of optical power meter 8 and determines whether there is an abnormal attenuation in the optical power compared to the length of optical fiber 5 to thereby determine whether the extension of optical fiber 5 is satisfactory or not. As a result, there are no problems, optical fiber 5 is connected at either ends to transmission optical link 3' of communication device 7 and to reception optical link 4' of communication device 9, as shown by white arrows to thereby operate the optical communication system. Like this, there are many cases in which when optical fiber 5 is reconnected to reception links 3', 4', the optical power fed to optical fiber 5 is adjusted in transmission link 3' in accordance with a quantity of optical power attenuation (in proportion to the length of optical fiber 5) transmitted through optical fiber 5. The adjustment of the fed optical power is generally performed by changing the current fed to the LED which is a light source in transmission optical link 3'.
FIG. 15 illustrates an circuit disposed in transmission link 3' for adjusting the optical power.
As shown in FIG. 15, distance selection switch 38 is switched manually in accordance with the length of optical fiber 5 or the communication distance. Switch 38 selectively switches current limiting resistors 35, 36, 37 different in resistance value disposed between the power source and LED 33 to change the light emission output from LED 33. Transistor 34 is turned on/off in accordance with a signal applied to its base terminal to thereby modulate light emitted by LED 33. Assume that the resistance values R.sub.L, R.sub.M, R.sub.H of resistors 35, 36, 37 have the relationship R.sub.H &lt;R.sub.M &lt;R.sub.L. Resistor 37 for a long distance, resistor 36 for an intermediate distance, and resistor 35 for a short distance are selected by switch 38 when the optical power transmitted through optical fiber 5 is high, immediate and low, respectively.
As just described above, the conventional techniques require works of extending optical fiber 5 on the spot, and connecting optical fiber 5 to the standard optical source 6 and power meter 8, and optical links 3', 4'. During extension and connection of such optical fiber 5, optical fiber 5 is exposed at each end to air. If the environment where the work is done is within a machine tool, an oil and/or dust exist. Therefore, even if the transmission characteristic of light is checked using optical power meter 8, the characteristic measured in the actual communication cannot often be obtained because the fiber end faces are rendered dirty due to the environment when optical fiber 5 is connected to optical link 3', 4'.
The connections of optical fiber 5 and optical links 3', 4' or between optical fiber 5 and standard light source 6, optical power meter 8 vary in transmission characteristic each time connection is made, so that the reading of optical power meter 8 as it is does not necessarily indicate the transmission characteristic in the actual communication.
One light emission diode (LED 33)used as the light emitting source in transmission optical link 3' may generally differ greatly in emission efficiency from the ideal one (forward current vs. light emission intensity) and the light intensity is influenced by the environmental temperature. Therefore, switch 38 of FIG. 15 must be switched and the optical power must be adjusted in consideration of the respective varying characteristics of the links. However, the conventional techniques cannot measure the individual characteristics of the links. Thus, the trouble with the conventional techniques is that the characteristics of links 3', 4' must be checked using another measuring instrument.
The photodetector of reception optical link 4' may differ in sensitivity from the ideal one, so that the reading optical power meter 8 as it is does not necessarily indicate the transmission characteristic in the actual use.
Optical fiber 5 extended on the spot may differ greatly from the ideal one in transmission performance when it is connected to the optical link, and the transmission characteristic to the optical fiber-optical link connection system cannot be evaluated correctly in the conventional technique.
The applicant has invented a device which is capable of performing extension and connection of an optical fiber without exposing it to a bad environment rich of oil and/or dust in a machine tool, as mentioned above. This device extends an optical fiber connected beforehand to an optical link, then performs a connecting work at an electric connector of the optical link which is less likely to be rendered dirty.
The present invention has been made in view such situation. It is an object of the present invention to provide a device which is capable of correctly evaluating the signal transmission performance in the actual communication in an optical communication system where the connection of an optical signal line to both control devices at the electric connectors of the optical link is performed as preprocessing.